1. Field of the Invention
The present invention relates generally to a nitride semiconductor device comprising a gallium nitride-based semiconductor layer and a process of manufacturing the same. A particular object of the present invention is to improve on the luminous efficiency of a nitride semiconductor light-emitting device comprising a gallium nitride-based semiconductor layer formed on a dissimilar substrate.
2. Related Art
In recent years, nitride semiconductor light-emitting devices comprising gallium nitride-based semiconductor layers have been put to practice use in a variety of fields and actually used in every aspect of daily life in the form of LEDs for emitting light in varying colors and at varying wavelengths such as blue light, green light, white light, and ultraviolet light. White LEDs in particular are a promising candidate for the coming generation of illumination devices that will take the position of fluorescent lamps. To achieve a nitride semi-conductor device with such properties as high efficiency and high luminance, however, there is still plenty of room for improvements.
In nitride semiconductor devices widely used so far in the art and comprising gallium nitride-based semiconductor layers, an n-type gallium nitride-based semiconductor layer, an active layer and a p-type gallium nitride-based semiconductor layer are stacked on the C-plane of a sapphire substrate. Why such nitride semiconductor devices have wide applications is that gallium nitride having satisfactory crystallographic properties is obtained because of relatively ready growth of gallium nitride on the C-plane of sapphire. For actual use of nitride semiconductor devices comprising such gallium nitride-based semiconductor layers, it is of importance that gallium nitride possess satisfactory crystallographic properties. The gallium nitride used to this end has a wurtzite crystal structure wherein the growth plane of gallium nitride is defined by the C-plane.
Referring generally to a nitride semiconductor quantum well prepared on a sapphire substrate, the well grows in the C-axis direction, and so large piezoelectric fields occur vertically within the growth plane. This in turn causes an emission energy level to be shifted toward a lower energy side as compared with the case of no electric field (quantum confined Stark effect) and electrons and holes to be spaced away from each other, offering problems such as drops of the probability of light emission and luminous efficiency.
To address such problems, JP-A 11-112029 discloses at pages 2-5 and illustrates in FIG. 9 an LED wherein for a device structure with reduced Stark effect on, for instance, gallium nitride, quantum wells are formed on the A-plane (2-1-10), M-plane (0-110) and R-plane (2-1-14) rather than on the C-plane (0001) of gallium nitride. P. Waltereit and seven others, “Nitride semiconductors free of electrostatic fields for efficient white light-emitting diodes, letters to nature”, NATURE, Aug. 24, 2000, vol. 406, pp. 865-868 reports that a GaN/AlGaN quantum well structure formed on an LiAlO2 (100) substrate grows in the M-axis direction and so can form a heterointerface with no electric field, whereby an emission peak energy is more enhanced than that of a sample on the C-axis, leading to an increase in the probability of radiative transition.
Since it is difficult to prepare high-quality LiAlO2 (100) substrates, however, it is vitally important to develop technologies for the formation of M-plane active layers on sapphire substrates. In addition, if the M-plane or the A-plane can arbitrarily be formed, it is then possible to provide a high-density M-plane active layer, a high-density A-plane active layer or a high-density active layer on a combined M- and A-plane. This is now expected to play an important role in the achievement of light-emitting devices having enhanced efficiency and increased output.
Still, much is desired for the luminance of gallium nitride-based LEDs, and for the output of semiconductor laser diodes (LDs). Decreases in luminous efficiency due to quantum confined Stark effect are a not-to-be-missed problem, and reductions or elimination of this effect poses a challenge. Thus, the achievement of an unheard-of device structure having higher luminous efficiency and capable of superseding existing LEDs and LDs are in great demand. A primary object of the present invention is to provide a novel device structure having high luminous efficiency, in which the decreases in luminous efficiency due to the aforesaid Stark effect are reduced or eliminated.